Aims: The hypothesis that magnetic fields influence breast cancer risk is based on the existence of a plausible mechanism related to the effect of magnetic fields on melatonin along with some limited epidemiologic data. The study has two specific goals. 1. To determine if residential exposure to magnetic fields, as assessed by wiring configuration coding and direct measurements is associated with the risk of breast cancer. 2. A secondary hypothesis is that particular combinations of the alternating current (AC) magnetic field and the direct current (DC) magnetic field, increase the risk of breast cancer. This hypothesis was prompted by observations of biologic effects at particular combinations of the AC and DC fields in several experimental systems. Procedures and techniques: The study is a case-control study nested within a cohort study of risk factors for breast and other cancers among approximately 54,000 African-American and Latino women in Los Angeles County. We enrolled cases of breast cancer diagnosed over the 5 year study period and controls randomly sampled from the cohort. We obtained an indirect measure of magnetic fields (wire mapping) on 744 cases and 703 controls and made in-home magnetic field measurements on 342 cases and 283 controls. We ended enrollment of subjects and wire mapping in December 2000. We have completed data checking and have begun analyses. Accomplishments: We have enrolled over 700 cases and controls. Data entry and checking are complete. Analyses have begun. We expect to complete manuscripts in FY2002.